As technology for coding moving-image data, coding schemes called MPEG-4 and H.264/AVC (hereinafter referred to as H.264) are known. In recent years, standardization work of a coding scheme called high efficiency video coding (HEVC) as the next generation standard is in progress. The HEVC requires larger computational complexity than the conventional H.264, but it is known that the HEVC achieves higher coding efficiency. In this coding scheme, the block configuration within a frame has a hierarchical structure having a higher degree of freedom than the conventional H.264 or the like and the number of candidates for block sizes increases. Hereinafter, a procedure of a determination process of the block configuration in HEVC reference software will be described.
In the HEVC reference software, an image region of an encoding target is divided into units of square blocks called largest coding units (LCUs) of a size of 64 pixels×64 pixels (hereinafter referred to as 64×64) having a hierarchical block configuration and encoding is performed for each LCU. In encoding, it is possible to set a smaller region as an encoding target by iterating a process of recursively dividing the LCU into four equal parts, up to a maximum of three times. When the LCU is 64×64, it is possible to divide a 64×64 region into 32×32, 16×16, and 8×8 regions of a quadtree structure obtained by recursively dividing the region into four parts. That is, the LCU of this case includes regions of four layers. Each of the blocks into which the LCU is divided is referred to as a coding unit (CU), and the LCU can be configured by combining CUs of different block sizes or CUs of the same block size.
FIG. 15 is a diagram illustrating an example in which the LCU is constituted of a combination of a plurality of CUs. In the example illustrated in FIG. 15, the LCU is constituted of a combination of two 32×32 CUs, seven 16×16 CUs, and four 8×8 CUs. When the moving-image data is encoded, it is possible to perform intra prediction or inter prediction and determine a prediction mode for each prediction unit (PU) obtained by further dividing each of the CUs constituting the LCU. In the determination of the block division prediction mode, an evaluation value called an RD cost expressed by the following Formula (1) is generally used.(Evaluation value)=D+λR  (1)
In Formula (1), D is an error between a restored signal and an original signal in a prediction mode, R is an information amount, and λ is a Lagrangian parameter. A prediction mode and a combination of CUs with which the evaluation value calculated in Formula (1) is minimized is determined as a final block configuration of the LCU in encoding. When the prediction mode is determined without a block configuration determination process of the LCU being optimized, the prediction mode is determined for each of combinations of CUs within the LCU. In this case, because the computation increases in the process of determining the block configuration of the LCU, the load of this process significantly increases in the entire encoding process and a heavy burden is imposed. Consequently, reducing the load necessary for the process of determining the block configuration of the LCU while suppressing the degradation of the coding efficiency is important from the viewpoint that the computational complexity of the entire encoding process is reduced and such a technique is desired.
To this end, a method for reducing the load by limiting candidates for block sizes serving as selection targets when the block configuration of an LCU is determined has been proposed (for example, Non-Patent Document 1). For example, there is a method for designating block sizes from the block size one level lower than the smallest block size of an adjacent block (PU) to the block size one level larger than the largest block size of the adjacent block as candidates when the block configuration of the LCU is determined.
FIGS. 16A and 16B are diagrams illustrating examples in which the block configuration in an encoding target block is determined based on the block configuration in an adjacent block. FIG. 16A illustrates the case in which the block configuration of the adjacent block includes three 32×32 blocks, three 16×16 blocks, and four 8×8 blocks. In this case, candidates for the block sizes when the block configuration of the encoding target block is determined become 64×64 to 8×8 as illustrated in FIG. 16A. FIG. 16B illustrates the case in which the block configuration of the adjacent block includes four 32×32 blocks. In this case, the candidates for the block sizes when the block configuration of the encoding target block is determined become 64×64 to 16×16 as illustrated in FIG. 16B.